Discharge lamps, and particularly high-power metal-halide discharge lamps, present problems in connection with reliable long-term seal of an electrical lead-through into a ceramic discharge vessel. Ceramic plugs are customarily used. There are many proposals for solutions to the problems. A pin or a tubular element of a metal, such as tungsten or molybdenum, is used as the electrical conductor. The plug may be of ceramic, and the pin or tube is melt-sealed by means of a glass melt or a melt ceramic into the plug. Alternatively, the lead-through may be directly sintered to the plug. The connection between the ceramic and the metal is not a secure bond however, so that the seal has a limited lifetime. It has also been proposed to use a cermet, which is a combination material formed of ceramic and metal, as the material for the plug--see U.S. Pat. Nos. 5,404,078, Bunk et al., and 5,592,049, Heider et al.
Plugs have been tested which comprise a plurality of layers of cermet with different relationships of metal to ceramic to provide for better matching of thermal coefficients of expansion. European EP 0 650 184 A1, Nagayama, to which U.S.-designated PCT/JP93/00959 corresponds, discloses a non-conductive cermet plug having axially arranged layers. This seal is very complex and uses a lead-through which has a thread, an outer metal disk or flange, and a metal or glass melt.
U.S. Pat. No. 4,602,956, Partlow et al., discloses a metal-halide discharge lamp having a ceramic discharge vessel. The electrode is carried in a lead-through which is formed as a disk of electrically conductive cermet. The electrode is sintered into the cermet. Additionally, the lead-through is surrounded by a ring-shaped stopper or plug of cermet which is connected with the ceramic discharge vessel, typically of aluminum oxide, by a glass melt. The glass melt, however, is corroded by aggressive components of the fill in the discharge lamps, particularly by the halides therein, so that the lifetime of such a lamp is rather short. Embedding the electrode in the cermet lead-through, additionally, leads to stresses which eventually may lead to fissures and cracks in the cermet. The diameter of the disk lead-through is quite large. The lead-through is electrically conductive and, thus, the discharge arc can flash back or arc back to the lead-through which would quickly lead to blackening of the discharge vessel.
U.S. Pat. No. 4,155,758, Evans, describes a special arrangement for a metal-halide lamp having a ceramic discharge vessel without an outer surrounding envelope. The lead-through is formed as a pin of electrically conductive cermet. The electrode is sintered into the cermet. The cermet pin in turn is sintered into a plug of aluminum oxide, and this plug is connected to the vessel by a glass melt. This arrangement also has the disadvantages above mentioned.
U.S. Pat. No. 5,424,609, Geven et al., describes a metal-halide discharge lamp with ceramic discharge vessel which requires an extremely long-drawn capillary tube of aluminum oxide as an inner plug element. A pin-like metallic lead-through is connected by a glass melt at the outer end in a melting zone. It is important that the melting zone is at a sufficiently low temperature. The lead-through pin can be made of two parts, in which the part facing the discharge can be made of an electrically conductive cermet, which contains carbide, silicide or a nitride. The sealing technology results in a large overall length of the discharge vessel, it is expensive to make and, also, uses the corrosion-susceptible glass melt. The gap between the capillary tube and the lead-through results in a comparatively large dead volume in which a substantial portion of the fill in the lamp may condense, so that a large quantity of fill is necessary. The aggressive fill has intensive contact with the corrosion-susceptible components in the sealing region. This technology can be used only in small power ratings, up to about 150 W, since, with larger inner diameters of the capillary tube, the actual difference in thermal expansion between the lead-through pin of cermet and the capillary tube would be too great.